We study the dynamics of a multimode VCSEL with an elliptical oxide aperture for datacom applications. We simulate the laser dynamics through a set of coupled rate equations for the modal components of the electric field and the carrier density, accounting for coherent mode mixing and spatial hole burning. Our simulations show what are the relevant frequency detuning configurations to control in order to improve noise performance. Simulations with NRZ PRBS performed in order to explore the applications of these devices in short-reach data transmission show potentially reachable transmission speeds of 65 Gbit/s.
We address the dynamics of a III-V/SiN external cavity hybrid laser with a frequency-selective mirror. Simulations through a set of time-delayed algebraic equations accounting for the narrowband SiN mirror reflectivity demonstrate that CW instability occurs when relaxation oscillations become resonant with the beating between longitudinal modes. In the CW unstable regime, we observe the emergence of frequency combs due to four wave mixing, characterized by a modulation of amplitude and frequency. We then characterize the resulting combs in terms of achievable bandwidth. These results are also confirmed through a more accurate time-domain traveling wave model.
In this manuscript, we employ a time-domain traveling-wave model with a coupled-mode theory to characterize the dynamic behavior of a mid-Infrared (MIR) Quantum Cascade Laser (QCL) in the Distributed-Feedback (DFB) configuration. Our investigation underscores the crucial influence of the linewidth enhancement factor (LEF) and spatial hole burning (SHB) on the single-mode behavior of DFB QCLs. Disregarding these factors leads to an overestimation of the range of pump currents granting single-mode emission and results in an inaccurate simulation of the multimodal dynamics of DFB QCLs. The numerical simulations presented in this work closely align with experimental observations, specifically focusing on a DFB QCL operating at a wavelength of 9.34 μm.
We present a project offered to Master students in Electronics Engineering or Communication and Computer Network integrating electronics and photonics. Teams, with different background knowledge, design independently an optical link following specifications provided by teachers.
We generalized the well-known Lugiato-Lefever Equation to unify the description of combs and localized structures formation in nonlinear optical systems such as Kerr micro-resonators (passive systems) and Quantum Cascade Lasers (QCL) (active systems). In particular this model was applied to the study of pattern formation in a unidirectional ring QCL driven by a coherent injected field. We showed the existence of Dissipative Solitons (DS) and Turing rolls associated to standard and harmonic Optical Frequency Combs (OFC) in the system. We also provided a proof of principle demonstration of the possibility to deterministically control the spectral properties of these OFC by switching-on one or more DS with suitable addressing pulses.
These results considerably increase the theoretical insight in chip-scale combs sources in the mid-infrared region of the electromagnetic spectrum for timely applications in the field of e.g. high resolution and/or time resolved molecular spectroscopy, long range and high bit rate wireless communications.
We present a new approach for the modelling of non-linear effects in silicon based ring resonators by coupling equations for Two-Photon-Absorption (TPA), Free-Carrier-Absortpion (FCA) and self-heating with the Shockley–Read–Hall theory involving trap-assisted recombination processes. SRH gives a non-linear carrier lifetime which is essential to fit model results with the experiments. The developed model is validated by comparison with experimental measurements performed on different ring types and it is employed in the design of rings with minimal non-linear effects for integration in ring-based Si PIC mirrors for high power hybrid III-V/Si tunable lasers.
We study the stability of a hybrid laser source consisting of a III-V reflective semiconductor optical amplifier (RSOA) edge-coupled to a silicon photonic mirror, based on two coupled high-Q microring resonators, providing a narrow band effective reflectivity. We simulate the laser dynamics through a model of time-delayed algebraic equations accounting for the frequency-selective mirror reflectivity, demonstrating single-mode emission, self pulsing, and turbulent regimes. Further, we identify the regions of higher CW operation in terms of bias current and laser detuning with respect to the reflectivity peak. Finally, we test the CW laser stability with respect to optical feedback, mimicking the effect of spurious back-reflections from the passive parts of the circuit, and demonstrate ultra-stable CW operation for a sizeable range of detuning.
We propose a generalization of the well-known Lugiato-Lefever Equation to unify the description of combs- and structures- forming nonlinear optical systems. This approach rigorously connects for the first time passive systems such as Kerr micro-resonators and active systems such as Quantum Cascade Lasers (QCLs) which were so far treated separately. The model effectively describes a unidirectional ring QCL driven by a coherent signal where we show the existence of temporal solitons and Turing rolls, previously identified only in Kerr micro-resonators, considerably increasing the theoretical insight and the technological potential of chip-scale comb sources.
Experiments on QD lasers grown on GaAs and on Si have revealed the quenching of the GS optical power as the current overcomes the ES threshold. A common technique to mitigate this quenching is the modulation p-doping, but an excessive p-doping level results in a deterioration of the GS optical power and threshold current. Theoretical models based on rate equations have ascribed the GS power quenching to the de-synchronization between the electron and hole dynamics. However, these approaches resort to phenomenological transport times. In this contribution, we study a 1.3 um QD laser grown on silicon by employing a drift-diffusion model for the transport of carriers across the SCH region. We show that the unbalance of electron and hole mobilities in the GaAs barriers is responsible for the GS quenching. The simulations also emphasize the existence of an optimum modulation p-doping level minimizing the GS threshold current, which we ascribe to electrostatic effects induced by this doping.
We developed a Time Domain Traveling Wave model to properly study the dynamics of a hybrid lasers realized by coupling a III-V Reflective Semiconductor Optical Amplifier with a Silicon Photonics mirror providing a narrow effective reflectivity (<10GHz). In free running operation mode, we show that for realistic values of the Henry factor stable single mode emission only occurs around the maximum of the reflectivity slope. Very interestingly for applications, in presence of optical-feedback, we access a regime of ultra-stability with respect to unwanted reflections or to self-oscillations triggered by a photon-photon resonance phenomenon.
By applying a recently proposed coupled-Bloch-mode approach, we have derived the resonance condition for the longitudinal modes of passive photonic crystal (PhC) line-defect cavities. We have derived simple expressions for the electric field depending on the size of the cavity and the order of the resonant mode. We have shown that, as the cavity becomes longer, the fundamental mode turns from FP-like to DFB-like and the fraction of its wavevector components within the light cone is gradually suppressed. Importantly, we have clarified the physical origin for this behaviour.
We study the coherent multi-mode phenomena in single section Quantum Dot (QD) lasers using a time domain traveling wave approach.
In the conventional Fabry-Perot configuration and close to the lasing threshold, we report a continuous wave solution (CW) instability consisting in several longitudinal modes turned on by the strong carrier grating due to the standing wave pattern. In this regime we found intervals of the bias current where the FP-QD laser spontaneously emits Optical Frequency Combs (OFC) as narrow, equally spaced, spectral lines with locked phases.
Interestingly, in the unidirectional ring configuration, where carrier grating due to standing wave pattern cannot take place, our simulations show the occurrence (at high pump current) of a new type of self-pulsing phenomenon leading to sub-picosecond pulses with THz repetition rate, multiple of the ring free spectral range. The linear stability analysis of the CW solution of the ring laser is in good agreement with the numerical simulation and it allows to establish an analogy between the observed self-pulsing regime and the well known Risken-Nummedal-Graham-Haken instability consisting in the amplification of the Rabi frequency of the system. Systematic simulations also indicate that, contrary to what happens for self-generation of OFC in FP laser, THz self-pulsing is robust and controllable over a wide range of bias currents, device lengths and degree of inhomogeneous gain broadening. Our results on FP lasers well agree with recent experimental evidences.
Optical Frequency Combs (OFC) generated by semiconductor lasers at optical communication wavelengths are promising laser sources for high capacity optical interconnects exploiting WDM techniques; very often they are integrated with Silicon Photonic integrated circuits to realize compact and low-cost transmitters. Quantum Dot (QD) or Quantum Dash (QDash) single section Fabry-Perot lasers have turned to be a good candidate for this application because they can generate a comb of self-locked optical lines using just one laser diode operating in CW and no saturable absorber section. In this talk we review the state-of-art of these devices and their applications, evidencing also the analogies with single section Quantum Cascade Lasers, that, as for QD and QDash lasers, generates optical combs in the mid-IR or THz range.
We will focus on the understanding of the physical effects that can explain the self-locking of the lasing lines and we will compare the self-locking mechanism in Quantum Dot and Quantum Well lasers. We will then present the numerical simulation tool we have developed to simulate the self-locking in Quantum Dot Fabry-Perot lasers. Our model is based on a Time-Domain Traveling-Wave (TDTW) approach that properly accounts for coherent radiation-matter interaction in the semiconductor active medium and includes the carrier grating generated by the optical standing wave pattern in the laser cavity. We show that the latter is the fundamental physical effect at the origin of the multi-wavelength spectrum appearing just above the laser threshold, but it is not enough for forcing the self-locking of the optical lines. The self-mode-locking regime associated with the emission of OFC is achieved for higher bias currents and it ascribed to nonlinear phase sensitive effects as Four Wave Mixing (FWM). To quantify the locking of the lines we have calculated some indicators that are obtained by the post processing of the calculated optical electric field of the laser output. These indicators are the RF spectrum at the beat note, the optical linewidth of the lasing lines and the Relative Intensity Noise (RIN) spectrum for both the total power and the power of each line. Varying the CW injected current above threshold we have observed three different regimes: in the first one, at low current, the laser is dominated by multi-wavelength emission with rather wide RF beat note and high low frequency RIN, this regime corresponds to an unlocked regime. In the optical spectrum we observe an optical line and side bands due to FWM components. In the second regime, at much higher current, the RF beat note is extremely narrow and the low frequency RIN of each line reduces significantly; in the optical spectrum the lines narrow and the side-bands disappear. This is a self-locked regime. In an intermediate current range, we have a transition regime where the state (locked or unlocked) depends on the initial conditions.
Our results explain in detail the behavior observed experimentally by different research groups and in different QD and Quantum Dash (QDash) devices.
Optical frequency combs generated by self mode-locking of single-section quantum dot based semiconductor lasers are ideal sources for applications in high capacity optical interconnects or high precision dual comb spectroscopy. We investigate a 1mm long InAs/InGaAs quantum dot semiconductor laser both experimentally and by simulations using a time-domain traveling-wave model. We observe that by increasing the injection current, the laser output exhibits an unlocked multi-mode behavior above the lasing threshold up to a certain current were the modes lock due to an internal non-linear effect in the active laser medium. This phase locking is experimentally and numerically observed by RF beat note line width analysis as well as by integrated relative intensity noise analysis. Both of these properties are significantly reduced above this locking threshold. The lowest experimentally measured RF line width amounts to 20 kHz, while for lower currents prior to the threshold the line width can be as high as hundreds of MHz. Our simulations confirm this threshold behavior and the simulated spectra are in good qualitative and quantitative agreement.
We compare the design of three different single mode laser structures consisting in a Reflective Semiconductor Optical Amplifier coupled to a silicon photonic external cavity mirror. The three designs differ for the mirror structure and are compared in terms of SOA power consumption and side mode suppression ratio (SMSR). Assuming then a Quantum Dot active material, we simulate the best laser design using a numerical model that includes the peculiar physical characteristics of the QD gain medium. The simulated QD laser CW characteristics are shown and discussed.
We report a systematic analysis of the phenomenon of self-generation of optical frequency combs in single section Fabry-Perot Quantum Dot lasers using a Time Domain Travelling Wave model. We show that the carriers grating due to the standing wave pattern (spatial hole burning) peculiar of Quantum Dots laser and the Four Wave Mixing are the key ingredients to explain spontaneous Optical Frequency Combs in these devices. Our results well agree with recent experimental evidences reported in semiconductor lasers based on Quantum Dots and Quantum Dashes active material and pave the way to the development of a simulation tool for the design of these comb laser sources for innovative applications in the field of high-data rate optical communications.
We investigate the spectrally-resolved relative intensity noise (RIN) of a dual state emitting quantum-dot (QD) laser in dependence on the laser biasing conditions. We study the RIN under free-running conditions as well as under external optical feedback (OFB). We ï¬nd an improvement in RIN of the free-running laser when ground-state (GS) and excited-state (ES) emit simultaneously as compared to a single-state emission. Furthermore, we ï¬nd an improvement in RIN under external OFB.
This work theoretically studies the impacts of the inhomogeneous broadening on the modulation dynamics of quantum dot lasers using a multi-population rate equation model. The modulation dynamics shows two distinct regimes depending on the energy separation between the GS and the ES. For broadenings smaller than the GS-ES separation, the K-factor increases while the damping factor offset, the differential gain and the gain compression factor decrease with the inhomogeneous broadening. For broadenings larger than the GS-ES separation, the damping factor offset keeps almost constant while the K-factor, the differential gain and the gain compression factor increases with the inhomogeneous broadening.
Thanks to optimized growth techniques, a high density of uniformly sized InAs quantum dots (QD) can be grown on
InP(113)B substrates. Low threshold currents obtained at 1.54 μm for broad area lasers are promising for the future. This
paper is a review of the recent progress toward the understanding of electronic properties, carrier dynamics and device
modelling in this system, taking into account materials and nanostructures properties. A first complete analysis of the
carrier dynamics is done by combining time-resolved photoluminescence experiments and a dynamic three-level model,
for the QD ground state (GS), the QD excited state (ES) and the wetting layer/barrier (WL). Auger coefficients for the
intradot assisted relaxation are determined. GS saturation is also introduced. The observed double laser emission for a
particular cavity length is explained by adding photon populations in the cavity with ES and GS resonant energies. Direct
carrier injection from the WL to the GS related to the weak carrier confinement in the QD is evidenced. In a final step,
this model is extended to QD GS and ES inhomogeneous broadening by adding multipopulation rate equations
(MPREM). The model is now able to reproduce the spectral behavior in InAs-InP QD lasers. The almost continuous
transition from the GS to the ES as a function of cavity length is then attributed to the large QD GS inhomogeneous
broadening comparable to the GS-ES lasing energy difference. Gain compression and Auger effects on the GS transition
are also be discussed.
Quantum dot (QD) lasers exhibit many useful properties such as low threshold current, temperature and feedback
insensitivity, chirpless behavior, and low linewidth enhancement factor. The aim of this paper is to investigate the lasing
spectra behaviour of InAs/InP(311B) QD lasers. In order to reach the standards of long-haul transmissions, 1.55μm
InAs QD lasers grown on InP substrate have been developed. More particularly, it has been demonstrated that the use of
the specific InP(113)B substrate orientation when combined with optimized growth techniques allows the growth of very
small (4 nm high) and dense (up to 1011cm-2) QD structures. Consequently, a model based on the multi-population rate
equations (MPRE) taking into account many cavity longitudinal modes for the calculation of the entire emission
spectrum has been developed. In order to include the inhomogeneous gain broadening of the QD ensemble, various dot
populations, each characterized by a ground state (GS) and an excited state (ES) average energy level have been
considered. It will be shown that the numerical results are in good agreement with the experimental ones, both for the
case of the double laser emission and for the effects of the homogeneous broadening on the lasing spectra. This
numerical investigation based on carrier dynamics is of prime importance for the optimization of low cost sources for
optical telecommunications as well as for a further improvement of QD laser performances at 1.55-μm on InP substrate,
as already demonstrated for InAs-GaAs QD lasers emitting at 1.3-μm.
InP based quantum dash active materials have been recently presented in the literature for their promising emission characteristics in the 1.55 μm wavelength range and for their broadband emission spectrum, that makes them interesting candidates as active material of semiconductor optical amplifiers and widely tunable lasers. The effect of p-type modulation doping in quantum well and quantum dot semiconductor active materials has been extensively studied in the literature as a technological possibility to increase the differential gain and the modulation bandwidth of semiconductor lasers, but, to our knowledge, it has never been considered for quantum dash lasers. In this work the effect of p-type modulation doping in a quantum-dash active material is theoretically analyzed, using a quasi-equilibrium model that accounts for the carrier distribution in the dashes of the ensemble, in the wetting-layer and in the barriers. We will show that the carrier distribution in these states significantly influences the efficacy of p-type modulation doping.
The dynamic behavior of three section DFB self-pulsating lasers, used as clock recovery device, is theoretically analyzed using a time domain traveling wave simulator. The effect of the bias operating conditions on the self-pulsation frequency is discussed through both static and dynamic analysis. Detailed results on the clock recovery dynamics and the jitter characteristics are presented injecting a realistic pseudo-random data stream.
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